1. Field of the Invention
The present invention relates generally to a hydraulic power transmission joint for use in distribution of the driving force of a motor vehicle, and more particularly to a hydraulic power transmission joint capable of changing over the torque transmission characteristics in response to rotational-speed differences between two power shafts through the coupling thereof.
2. Description of the Related Arts
Conventional hydraulic power transmission joints are known from, e.g., U.S. Pat. Nos. 5,706,658 and 5,983,635.
To transmit torque in response to rotational-speed differences between two shafts, the hydraulic power transmission joint comprises:
a cam housing interposed between the input and output shafts and coupled to one of the shafts, the cam housing being provided with a cam face having two or more raised portions formed on its internal side; PA1 a rotor coupled to the other of the shafts and rotatably housed in the cam housing, the rotor having a plurality of axially extending plunger chambers; PA1 a plurality of plungers each reciprocatively accommodated in each of the plurality of plunger chambers under a pressing force of a return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the two shafts; PA1 an intake/discharge hole formed in the rotor and leading to the plurality of plunger chambers; PA1 a rotary valve being in rotatable sliding contact with an end face of the rotor, the rotary valve being positioned relative to the rotor in a predetermined relationship, the rotary valve having on its surface a plurality of intake ports and a plurality of discharge ports acting respectively as intake valves and discharge valves depending on a positional relationship relative to the intake/discharge hole; PA1 flow resistance generating means for generating flow resistance as a result of flow of oil discharged by the operation of the plungers. PA1 r.multidot..omega..sup.2 is an acceleration; and PA1 V is a vehicle velocity. PA1 a cam housing interposed between the input and output shafts capable of relative rotations and coupled to one of the shafts, the cam housing being provided with a cam face having two or more raised portions formed on its internal side; PA1 a rotor coupled to the other of the shafts and rotatably housed in the cam housing, the rotor having a plurality of axially extending plunger chambers; PA1 a plurality of plungers each reciprocatively accommodated in each of the plurality of plunger chambers under a pressing force of a return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the two shafts; PA1 an intake/discharge hole formed in the rotor and leading to the plurality of plunger chambers; PA1 a rotary valve being in rotatable sliding contact with an end face of the rotor, the rotary valve being positioned relative to the rotor in a predetermined relationship, the rotary valve having on its surface a plurality of intake ports and a plurality of discharge ports acting respectively as intake valves and discharge valves depending on a positional relationship relative to the intake/discharge hole; PA1 an orifice generating a flow resistance as a result of flow of oil discharged by the operations of the plungers. PA1 an accommodation hole formed outside of the discharge ports of the rotary valve; PA1 a weight member received in the accommodation hole rockably around a center of rotation by the action of a centrifugal force; PA1 a drain hole for allowing a communication between the accommodation hole and the discharge ports; PA1 a ball located under the weight member for blocking the drain hole, the ball serving to open the drain hole when the weight member rocks by a centrifugal force; and PA1 a spring for urging the end portion of the weight member opposite to the center of rotation, the spring setting a predetermined rotational-speed difference as a torque characteristic changeover point.
In the event that tires having different diameters are mounted on the front and rear shafts with use of such a hydraulic power transmission joint, the rotational-speed difference and torque will increase accordingly as the vehicle velocity rises, and resultant cumulation of the vehicle front and rear differential torque may cause an increase in the running resistance. To solve such a problem, a torque characteristic shifting mechanism of FIG. 1 is proposed which includes therein a weight portion which is displaced against a retainer spring force in response to a vehicle velocity (centrifugal force) to thereby relieve the internal pressure to change over the transmission torque. FIG. 1 is a partial sectional view of a rotary valve of the hydraulic power transmission joint provided with the torque characteristic shifting mechanism. A rotary valve 102 is rotatably housed in a cam housing 101. The rotary valve 102 has on its outer periphery a positioning protrusion 102A which engages with a notch 101A formed in the inner periphery of the cam housing 101. The rotary valve 102 is provided with discharge ports 103 and intake ports 104 which are formed alternately in the circumferential direction, with the intake ports 104 leading to intake passages 105 extending to the outer peripheral surface of the rotary valve 102. An accommodation hole 106 is formed at the outer periphery of the rotary valve 102 outside the discharge ports 104. The accommodation hole 106 is in the shape of a circumferentially elongated hole having a raised portion formed on its bottom. The accommodation hole 106 accommodates a weight member 107 in such a manner as to be displaceable in the radial direction (centrifugal direction). The weight member 107 has a bottom which is shaped so as to correspond to the bottom of the accommodation hole 106 and which is provided with a recessed portion. The top of the weight member 107 is provided with spring accommodation holes 110 and 111 accommodating therein springs 108 and 109, respectively. A drain hole 112 is interposed between the accommodation hole 106 and the discharge port 103 so that the accommodation hole 106 and the discharge port 133 can communicate with each other by way of the drain hole 112. The drain hole 112 has at its exit an accommodation groove 114 for receiving a ball 113. The ball 113 is received in the accommodation groove 114 and normally blocks the drain hole 112. A predetermined gap is formed between the weight member 107 and the accommodation hole 106. The springs 108 and 109 press the weight member 107 by a given spring force. The ball 113 received in the accommodation groove 114 formed at the exit of the drain hole 112 blocks the drain hole 112 by the spring force of the springs 108 and 109, and when a predetermined vehicle velocity is reached and the weight member 107 is displaced in the centrifugal direction by the centrifugal force, the ball 113 opens the drain hole 112. In this manner, the conventional torque characteristic shifting mechanism allows the weight member 107 to be displaced against the springs 108 and 109 in response to the vehicle velocity (centrifugal force) so that the ball 113 opens the drain hole 112 to relieve the internal pressure, thereby rendering the torque .DELTA.T variable as seen in FIG. 2.
In FIG. 2, a curve a represents a torque characteristic obtained when the vehicle velocity V has reached a predetermined vehicle velocity, and a curve b represents a torque characteristic obtained when the vehicle velocity V has reached a predetermined vehicle velocity V2. The transmission torque lowers depending on the vehicle velocity V as indicated by an arrow c, thereby preventing the running resistance from increasing.
Referring then to FIG. 3, description will be made of the balance at a point where the torque characteristic is shifted. Arrows d and e denote a spring force P of the springs 108 and 109, an arrow f denotes a centrifugal force (mr .omega..sup.2) acting on the weight member 107, and an arrow g denotes a thrust-up force (.DELTA.P.multidot.A) by the internal oil pressure thrusting up the weight member 107 via the ball 113 in the centrifugal direction. A represents the area of contact of the ball 113 with the drain hole 112, and .DELTA.P represents a discharge pressure. The discharge pressure .DELTA.P is proportional to a transmission torque .DELTA.T (.DELTA.P.varies..DELTA.T). The balance at the point (indicated by the vehicle velocity) at which the torque characteristic is changed over is therefore given as EQU 2P-m.multidot.r.multidot..omega..sup.2 -.DELTA.P.multidot.A=0, .omega..sup.2.varies.V.sup.2
where m is a mass of the weight member;
Such a conventional hydraulic power transmission joint allowed the springs to press the opposed ends of the weight member, with the balance at the torque characteristic shifting point being given as EQU 2P-m.multidot.r.multidot..omega..sup.2 -.DELTA.P.multidot.A=0
hence it suffered deficiencies which follow.
First, although the balance expression results in 2P=.DELTA.P.multidot.A when the vehicle velocity V is zero, a condition .DELTA.P.gtoreq.P.sub.0 (P.sub.0 is a predetermined pressure required for keeping the drain hole closed) needs to be satisfied in order to prevent any early drain, and it is also desirable to increase the area of contact A as much as possible with the aim of improving the fuel efficiency after shifting and of preventing any increase in the running resistance.
To acquire a larger contact area, the spring force has also to be increased. Due to the relationship that the spring force balances directly with the area of contact A, however, larger spring accommodation spaces are needed, resulting in the joint having a larger external diameter.
In the condition that .DELTA.P.multidot.A is constant (.DELTA.P.multidot.A=C), the balance expression is given as EQU 2P=m.multidot.r.multidot..omega..sup.2 +C
namely, EQU .omega..sup.2 =(2P.multidot.C)/m.multidot.r
For the purpose of improving the fuel efficiency after shifting and of suppressing any increase in the running resistance as described hereinabove, it is preferred to reduce the vehicle velocity V as much as possible. If the spring force P is constant, then it is preferred that the weight member have as large a mass m as possible. Due to the direct relationship of balance between m and other load, however, the overall length and the external diameter of the joint will increase, resulting in an enlarged joint.
In addition, due to the use of a single weight member for shifting the torque characteristic, the amount of torque reduction is apt to become larger after the torque shifting, which may adversely affect the variances in vehicle behaviors.